Electrophotography forms the technical basis for various well-known imaging processes, including photocopying and some forms of laser printing. Electrophotographic imaging processes typically involve the use of a reusable, light sensitive, temporary image receptor, known as a photoreceptor, in the process of producing an electrophotographic image on a final, permanent image receptor. A representative electrophotographic process involves a series of steps to produce an image on a photoreceptor, including charging, exposure, development, transfer, fusing, cleaning, and erasure.
In the charging step, a photoreceptor is covered with charge of a desired polarity, either negative or positive, typically with a charging device, such as a corona or charging roller. In the exposure step, an optical system, typically a laser scanner or light-emitting diode array, forms a latent image by selectively exposing the photoreceptor to electromagnetic radiation, thereby discharging the charged surface of the photoreceptor in an imagewise manner corresponding to the desired image to be formed on the final image receptor. The electromagnetic radiation, which may also be referred to as “light” or actinic radiation, may include infrared radiation, visible light, and ultraviolet radiation, for example.
In the development step, toner particles of the appropriate polarity are generally brought into contact with the latent image on the photoreceptor, typically using an electrically-biased development roller to bring the charged toner particles into close proximity to the photoreceptive element. The polarity of the development roller should be the same as that of the toner particles and the electrostatic bias potential on the development roller should be higher than the potential of the imagewise discharged surface of the photoreceptor so that the toner particles migrate to the photoreceptor and selectively develop the latent image via electrostatic forces, forming a toned image on the photoreceptor.
In the transfer step, the toned image is transferred from the photoreceptor to the desired final image receptor. An intermediate transfer element is sometimes used to effect transfer of the toned image from the photoreceptor with subsequent transfer of the toned image to a final image receptor. The transfer of an image typically occurs by either elastomeric assist (also referred to herein as “adhesive transfer”) or electrostatic assist (also referred to herein as “electrostatic transfer”).
Elastomeric assist or adhesive transfer refers generally to a process in which the transfer of an image is primarily caused by balancing the relative energies between the ink, a photoreceptor surface and a temporary carrier surface or medium for the toner. The effectiveness of such elastomeric assist or adhesive transfer is controlled by several variables including surface energy, temperature, pressure, and toner rheology. An exemplary elastomeric assist/adhesive image transfer process is described in U.S. Pat. No. 5,916,718.
Electrostatic assist or electrostatic transfer refers generally to a process in which transfer of an image is primarily affected by electrostatic forces or charge differential phenomena between the receptor surface and the temporary carrier surface or medium for the toner. Electrostatic transfer may be influenced by surface energy, temperature, and pressure, but the primary driving forces causing the toner image to be transferred to the final substrate are electrostatic forces. An exemplary electrostatic transfer process is described in U.S. Pat. No. 4,420,244.
In the fusing step, the toned image on the final image receptor is heated to soften or melt the toner particles, thereby fusing the toned image to the final receptor. An alternative fusing method involves fixing the toner to the final receptor under high pressure with or without heat. In the cleaning step, residual toner remaining on the photoreceptor is removed. Finally, in the erasing step, the photoreceptor charge is reduced to a substantially uniformly low value by exposure to light of a particular wavelength band, thereby removing remnants of the original latent image and preparing the photoreceptor for the next imaging cycle.
Two types of toner that are in widespread, commercial use for electrophotographic processes include liquid toners and dry toners. The term “dry” does not mean that the dry toner is totally free of any liquid constituents, but connotes that the toner particles do not contain any significant amount of solvent, e.g., typically less than 10 weight percent solvent (generally, dry toner is as dry as is reasonably practical in terms of solvent content), and are capable of carrying a triboelectric charge. This distinguishes dry toner particles from liquid toner particles.
A typical liquid toner composition generally includes toner particles suspended or dispersed in a liquid carrier. The liquid carrier is typically a nonconductive dispersant, to avoid discharging the latent electrostatic image. Liquid toner particles are generally solvated to some degree in the liquid carrier (or carrier liquid), typically in more than 50 weight percent of a low polarity, low dielectric constant, substantially nonaqueous carrier solvent. Liquid toner particles are generally chemically charged using polar groups that dissociate in the carrier solvent, but do not carry a triboelectric charge while solvated and/or dispersed in the liquid carrier. Liquid toner particles are also typically smaller than dry toner particles, ranging from about 5 microns to sub-micron. The liquid toner composition can vary greatly with the type of transfer used because liquid toner particles used in adhesive transfer imaging processes must be “film-formed” and have adhesive properties after development on the photoreceptor, while liquid toners used in electrostatic transfer imaging processes must remain as distinct charged particles after development on the photoreceptor.
Photoreceptors generally have a photoconductive layer that transports charge (either by an electron transfer or charge transfer mechanism) when the photoconductive layer is exposed to activating electromagnetic radiation or light. The photoconductive layer is generally affixed to an electroconductive support, such as a conductive drum or an insulative substrate that is vapor coated with aluminum or another conductor. The surface of the photoreceptor can be either negatively or positively charged so that when activating electromagnetic radiation strikes certain regions of the photoconductive layer, charge is conducted through the photoreceptor to neutralize, dissipate or reduce the surface potential in those activated regions. In order to achieve certain performance characteristics of the photoreceptor, it is advantageous for the charge on the photoreceptor surface to be maintained within certain ranges, even after extended use of the photoreceptor.
An optional barrier layer may be used over the photoconductive layer to protect the photoconductive layer and thereby extend the service life of the photoconductive layer. Other layers, such as adhesive layers, priming layers, or charge injection blocking layers, are also used in some photoreceptors. These layers may either be incorporated into the photoreceptor material chemical formulation, or may be applied to the photoreceptor substrate prior to the application of the photoreceptive layer or may be applied over the top of photoreceptive layer. A permanently bonded release layer may also be used on the surface of the photoreceptor to facilitate transfer of the image from the photoreceptor to either the final substrate, such as paper, or to an intermediate transfer element, particularly when an adhesive transfer process is used. U.S. Pat. No. 5,733,698 describes an exemplary permanently bonded release layer suitable for use in imaging processes using adhesive transfer.
The photoreceptors used in electrophotographic processes, such as those described above, tend to become stressed or fatigued after numerous printing cycles due to the repetitive charging and discharging of the photoreceptive surface. This is true for printing processes that use either liquid or dry toner. One of the indicators of photoreceptor fatigue is that the value of the charge on a fatigued photoreceptor surface is lower than the charge on the surface of a new or unstressed photoreceptor when subjected to the same charging conditions from a charging device. This reduced charge on the photoreceptor surface may be caused by an inability to establish the charge-up voltage on the photoreceptor surface with a fixed excitation by the charging device (i.e., the charge acceptance of the photoreceptor surface decreases as a function of time). The reduced photoreceptor surface charge my also be caused by an inability of the photoreceptor surface to hold or maintain the charge-up voltage for a certain period of time (i.e., the dark decay of the photoreceptor surface increases with repeated use of the photoreceptor). In these cases where a photoreceptor cannot accept and/or maintain a desired surface charge as it ages, the printed images will begin to exhibit a background stain or “ghosting” effect. When this occurs, the user will typically discard the aged photoreceptor and replace it with a new photoreceptor that is capable of again accepting and maintaining a specified charge-up voltage. However, there are techniques in the art that have been used to extend the life of a photoreceptor.
One approach that may be used to extend the useful life of a photoreceptor is to increase the voltage provided by the charging device. Ideally, this voltage increase will reestablish a desired surface charge on the photoreceptor surface to thereby improve the print quality. To determine the necessary increase in the charging device voltage, historical data is often collected regarding the photoreceptor performance, which can be plotted or recorded to predict the performance of similar photoreceptors when subjected to the same conditions. The photoreceptor performance data is often measured with an electrostatic voltage probe near the photoreceptor. The voltage measurements can then be sent to a processor for calculation of any adjustments that need to be made to the charging device voltage. One drawback to this technique is that the electrostatic voltage sensor heads or devices are relatively large as compared to the small amount of space available inside a printer. In addition, electrostatic voltmeter systems are often relatively costly. In a four-channel color printing machine, four voltmeter systems are needed to monitor the surface charge on four different photoreceptors (one for each color) during printing, which further increases the space needed within a printing device and increases the system costs. It is therefore desirable to provide an improved method and system for measuring and adjusting the surface voltage of a photoreceptor. It is further desirable that such methods and systems will use accurate measurement equipment that is relatively small and inexpensive.